Sounds like you have an egg eater. Once they get a taste for egg it's very hard to get them to stop. She'll also teach this behavior to your other chickens.
How to prevent it and stop egg eaters.
-Keeping the boxes in a dark, quite place. That way she can't see the egg when they are laid.
-Collecting eggs throughout the day. When a nest gets full of eggs, it's easy for them to bump into each other and crack. This is how most egg eaters start.
-Roll-a-way nesting boxes. These as the name says, roll the eggs were the hens can't reach. It also prevents broodiness.
-Giving your chickens calcium. Your hen may have a calcium or protein deficiency and eating her eggs is the only way she can get it. Low calcium levels also affect how hard the shells are. Make sure you are giving your girls good quality layer feed and oyster shells.
-Some people fill the eggs with mustard (which most chickens hate). This way when a chicken breaks open and tries to eat it, they get a nasty tasting surprise. You may need to fill more then one egg.
-Culling. If you are tired of not getting any eggs and simply can not get your chicken to stop you may just have to cull.
To catch the culprit, you can put a camera in your coop. Look for any chickens that may have yellow or sticky on their beaks and feathers.
